During the first years after a new major version is released, support for newer hardware is added.
After a number of years (and the next major version has been released) support for newer hardware is no longer added, unless it requires limited effort to do so. For RHEL 6 this is the case since May 2017.

I agree. If the hardware is from an enterprise vendor, they will generally provide a support matrix, however that is not always accurate. For example, we recently certified the HP ProLiant DL380 Gen 10, which HP stated was fully supported on EL7.3. During the certification process, it was noted that the 10GB network adapters that shipped with the unit would not work with the native driver provided with EL7.3 so our policy is a minimum of EL7.4 for these units.